1. Field of the Invention
The present invention relates to a circuit for preventing corrosion of a contact, the circuit having a function of applying current to and destroying an oxide layer developed by corrosion on contacts at a switch or a connector.
2. Description of the Related Art
Contacts such as those of a switch or a connector have been made of a metal material excellent in electric conduction so as to reduce a contact resistance on electric connection. There is a fear that such contacts may increase in contact resistance because a surface of a contact part is oxidized when a switch is turned off for disconnection. Further, when a contact is connected for turning on, there is a fear that a surface of a part exposed around the contact part may be oxidized, the oxide which is then caught in the contact part, thereby causing a slight sliding wear resulting in an increased contact resistance. If a contact state and a non-contact state are appropriately repeated and a relatively large current is allowed to flow during the contact state, such a current may be used to produce heat to remove the oxide, thereby preventing an increase in contact resistance even after the contact resistance is increased due to oxidation of contacts.
With regard to an input to an electronic device, it is in general not necessary to allow large current, which can prevent corrosion of a contact, to constantly flow into contacts. An intermittent flow of a large current may contribute to malfunctions due to noise. In addition, allowing a large current to flow into contacts may result in a greatly reduced electric life of contacts or adhesion of contacts. In order to solve these problems, JP-A-Hei.2-297818 discloses the following current control device. The device detects contact resistance of the contacts. When the contact resistance of a contact is equal to or larger than a predetermined reference value, the device allows a large current between the contacts.
FIG. 9 is a reprinted drawing of FIG. 1 of JP-A-Hei.2-297818. One end of a contact 1 such as a closing switch is connected to a +V power source. The other end of the contact 1 is grounded via a resistor 2 to a primary side of a photo coupler 3 (light-emitting diode). A secondary side of the photo coupler 3 (photo transistor) is connected between the +V power source and the ground via a resistor 4. The photo coupler 3 is turned on and off in accordance with opening and closing of the contact 1. On and off signals of the photo coupler 3 are output to a control circuit 5. A transistor 6 is connected via a resistor 7 to a series circuit of the resistor 2 and the primary side of the photo coupler 3, in parallel.
A detection circuit 16 detects whether or not a contact resistance of the contact 1 exceeds a certain value. The detection circuit 16 includes resistors 17, 18, 19, 20 and an operational amplifier 21. The resistors 17 and 18 are connected in series between the +V power source and the ground. A series circuit of the resistors 19 and 20 is connected in parallel to the series circuit of the resistor 2 to a primary side of the photo coupler 3. A connecting point P1 between the resistor 17 and the resistor 18 is connected to a non-inverting input terminal of the operational amplifier 21. An inverting input terminal of the operational amplifier 21 is connected to a connecting point P2 between the resistor 19 and the resistor 20. Thus, a voltage of Va produced at both ends of the resistor 18 by dividing a voltage of the +V power source by the resistors 17 and 18 is supplied to the non-inverting input terminal of the operational amplifier 21. Further, a voltage of Vb at both ends of the resistor 20 determined by the contact resistance of the contact 1 and the resistors 19 and 20 is supplied to the inverting input terminal thereof. Then, an output signal of the operational amplifier 21 activates a base of the transistor 6, which allows a load current I2 for removing disturbances flowing into the contact 1.
When the contact 1 is closed, a current I1 flows into the primary side of the photo coupler 3, so that the photo coupler 3 is operated and resultant signals are supplied to the control circuit 5. At this time, according to closing of the contact 1, the +V power source is supplied to the resistors 19 and 20 via the contact 1. Thus, voltage is generated on both ends of the resistor 20 according to the contact resistance of the contact 1. This voltage Vb on both ends thereof is supplied to the inverting input terminal of the operational amplifier 21. In this instance, the operational amplifier 21 compares the voltage Va with the voltage Vb to judge whether or not the contact resistance of the contact 1 is larger than the predetermined reference value.
If the contact resistance of the contact 1 becomes larger than the reference value due to generation of an insulating layer, Va is larger than Vb (Va>Vb). Thus, an output of the operational amplifier 21 becomes “H,” and the transistor 6 is turned on so as to allow the load current I2 to flow via the series circuit of the resistor 7 and the transistor 6. As a result, a contact current I0=I1+I2. Since a current flowing through the contact 1 increases by a value of I2 greater than a usual value, it is expected that the insulating layer between the contacts is destroyed by Joule heat so as to reduce the contact resistance.
Also, U.S. Pat. No. 5,523,633 discloses a circuit for preventing corrosion of a switch for large current. The switch allows a large current in a pulse shape during a period in which a contact of the switch is turned on, when the switch for large current is employed in a low-current system such as electronic control units. In addition, JP-A-Hei.7-14463 discloses a device for discriminating contact signals. The device allows a corrosion-prevention current in a pulse shape to flow periodically by means of charge and discharge into a condenser. JP-A-2002-343171 also discloses a device for preventing corrosion of a contact of a switch. The device flows large current for preventing corrosion for at least a predetermined holding time from a time point where the contact of the switch is changed from an opened state to a closed state. When the contact of the switch is in the opened state, the device decreases an impedance of an input signal line connected to the contact.